With proliferation of mobile devices such as smart phones, netbooks and tablet computers, there is a growing desire to obtain accurate location information of such devices because the information is of much use. Wireless positioning involves obtaining wireless signals and processing the signals into a location estimate. Typical information used for positioning includes Global Positioning System (GPS) signals, Received Signal Strength Indicator (RSSI), Angle of Arrival (AOA), Time of Arrival (TOA), Time Difference of Arrival (TDOA), and Doppler shift. This information is often processed to find the position of a wireless device. For example, triangulation is used where multiple range or angle measurements from known positions are used to calculate the position of a device.
One of the sources of errors in wireless positioning is multipath propagation. Multipath propagation occurs when a signal takes different paths when propagating from a source to a destination receiver. While the signal is traveling, objects get in the way and cause the signal to bounce in different directions before getting to the receiver. As a result, some of the signal will be delayed and travel longer paths to the receiver. In other instances there is no direct line of sight because an object is completely blocking and any received signals occur only due to multipath propagation. These effects cause errors in GPS data, RSSI, AOA, TOA, TDOA and Doppler shift. The computed position of the device using common techniques such as triangulation will therefore be incorrect.
Location-aware technologies compute the location of an object. These systems differ in terms of accuracy, coverage, cost of installation, and maintenance. GPS systems use satellite signals and work in outdoor environments. GPS systems, however, require direct line of sight and do not work in an indoor environment. Cell tower triangulation is another method that uses signals from cellular towers to locate a wireless user. This method is also limited in accuracy and reliability because of the coarse number of cell towers from a particular service provider that a mobile user can communicate with, as well as multipath issues.
Systems have been developed in the past that use the strength of wireless access point beacon signals in an outdoor environment to calculate the position of a mobile user. One technique is to create a database of wireless beacons and use that information together with the amplitude of beacons signals received by a mobile device to compute the location of the mobile device. Other techniques use radio frequency (RF) wireless signal strength information and triangulation to locate objects in an indoor environment. However, these methods provide poor indoor positioning accuracy because RF signal amplitude is greatly affected by metal objects, reflective surfaces, multipath, dead-spots, noise and interference.